This invention relates generally to consistometer container assemblies and methods of using them with a substance to be tested therein and more particularly, but not by way of limitation, to a positive stirring consistometer container assembly (hereinafter referred to as a "cup") and method of handling a thixotropic substance which is initially flowable but subsequently hardens into a solid attached to parts of the cup.
In the oil and gas industry, different fluids are used for various purposes in drilling and completing as well. For example, batches of cement slurry must be mixed and pumped into the well for cementing the casing into the well bore. The cement is generally pumped through the casing and up the annulus between the well casing and the well bore to create the necessary bond.
Because different batches of fluids (e.g., cement slurriers) can have different characteristics which affect how the fluids perform in the high temperature and high pressure environments found downhole, there is the need for equipment which can test a fluid sample prior to the fluid being pumped downhole so that one can determine if that particular batch of fluid has the proper characteristics for the particular situation. Such a type of equipment is known to the art, as exemplified by a high temperature-high pressure consistometer provided by Halliburton Services or one provided by Chandler Engineering.
Prior to the commencement of an actual cement job, for example, a particular type of slurry composition proposed to be used on the job is thoroughly tested in a cement testing laboratory, such as one where the aforementioned Halliburton Services or Chandler Engineering consistometer is used. One of the major areas of testing concerns the fluid life and pumpability of the cement slurry. The ultimate objective to be achieved by the test is to insure that the cement slurry to be used can be placed with reasonable time safety. At least one property indicating whether the particular cement slurry will achieve this objective can be obtained through a thickening time test performed via the consistometer.
Although the purpose of such a thickening time test is to simulate the pumping of the cement slurry in the downhole environment to test at least one of the characteristics of the slurry (particularly, the total fluid time of the slurry), the thickening time test which has been performed with at least one type of prior art consistometer cup provides only a relatively small amount of shear to the entire volume of the slurry tested. This prior art cup includes a container for holding the cement slurry and a paddle disposed in the container and relative to which the container rotates. The cement slurry at the interface of this container experiences shear, but the cement slurry near the center of the container experiences very little shear. This phenomenon can contribute to the shortcomings of such a prior art cup whereby reliable and repeatable results are difficult to obtain. It is also difficult to adequately test special or problem blends, such as thixotropic compositions. These shortcomings can result in seemingly inconsistent data obtained from identical tests performed on similarly constituted pilot and field blends, which can result in extensive and expensive expenditures of laboratory time and manpower.
One example of these shortcomings is manifested in the testing of thixotropic substances, such as a cement slurry having a rapidly increasing apparent viscoisty at decreasing shear rate and having a high rate of static gel strength increase imparted by adding thixotropic cross-linked materials or as a by-product of gas generating additives or as a result of a high concentration of water absorbing materials. Such thixotropic substances are even more sensitive to shear than conventional cement slurries, for example. Because of this, a reliable measurement of conventional thickening time is hard to obtain when only a small amount of shear is imparted to the thixotropic substance. Such thickening times obtained by using prior art cups have been found to indicate a shorter thickening time or a high degree of premature gelling when such indications are not in fact accurate descriptions of the particular substance tested. This has resulted in such blends being chemically modified to have their thickening times retarded and thier viscosities lowered when in actuality such additions or modifications were not needed, as would have been indicated if a more reliable thickening time had been measured. These modifications, by which the blend is over-retarded and over-dispersed, can result in delayed compressive strength development once the actual field blend is pumped into the well bore so that it takes longer for the cement to properly bond. This increases the "waiting on cement" time and, correspondingly, the cost; it also decreases the thixotropic properties for which the slurry was formulated.
As another example of the shortcomings of such prior art consistometer cups, it has been found that repeatable test results between seemingly identical tests on identical slurries under high pressure applications have a great deal of variation.
From the foregoing, it is apparent that there is the need for an improved consistometer cup which can be used with existing test apparatus to more reliably and repeatedly obtain thickening time measurements of substances, such as conventional and special cement blends. A concomitant method of using such a cup to more easily handle the test substance is also needed.